1. Field of the Invention
The present invention relates to the fixation of medullary nails within bones, and more particularly, to a target device for the proximal and distal locking of medullary nails without X-rays.
2. Description of the Related Art
In order to prevent additional radiation loading of the operator, target devices for setting of locking screws in the medullary nail inserted in the bone are known. These target devices are rigidly connected with the medullary nail. The target device generally has guide bores through which the bone bore hole and the locking screw can be introduced so as to be exactly aligned with the contours of the bore hole and with the channels in the medullary nail.
Initially, however, only the proximal target device was fixedly connected with the medullary nail and a separate target device for placement of the distal locking screws had to be aligned by observing the same under X-ray. The current state of the art indicates that there are target devices which provide an additional fixed connection between the proximal and distal target devices.
Thus, according to DE 42 40 277 A1, an auxiliary rail is provided with guide holes for a bone drill to be applied and is screwed to the proximal end of the medullary nail. The guide holes are introduced in the auxiliary rail with a defined spacing analogous to the medullary nail. The auxiliary rail is fixed in its position relative to the bone in the area of the distal guide holes by means of fastening pins, e.g., Kirschner wire, and also with an intermediary of an additional bridge stirrup or clip.
DE 296 08 071 U1 goes a step further with respect to the fixing of the distal target device in that a spacer is provided in addition to a longitudinal target rail with which the distal target device is held at a predetermined distance from the proximal target device. The bore holes are aligned relative to the bore holes in the medullary nail by means of the spacer in a drill template fastened to the target rail. The Applicant's company manufactures a similar target arrangement in which the proximal and distal target device comprises, in each instance, a U-shaped part, wherein guide bores are introduced in the widened end areas of the legs of the U-shaped parts, the medullary nail is fixedly connected with the proximal target device via a crosspiece and an adjustable spacer is provided between the proximal and distal target device. The matching position of the guide bores of the distal target device with respect to the medullary nail channels which receive the locking screws is ensured by an adjustable and lockable spacer element which is guided through a bore in the bone so that it must rest against the medullary nail in the final position.
The alignment of the distal bore holes in the auxiliary rail and in the medullary nail is to be achieved according to DE 42 40 277 A1 by setting a thinner fastening pin through one of the bore holes beforehand followed by fixation by means of additional fastening pins anchored in the bone. The arrangement of a spacer element which is fastened to the distal target device and brought into contact with the medullary nail already offers substantially greater security for maintaining the alignment of the communicating bore holes.
However, the following problem occurs in the latter solution: When the medullary nail bends during insertion into the un-drilled bone in the anterior or posterior direction, the distal target device 4 is lifted along with it via the spacer element 1 (FIG. 1) and pivots about the joint 3 at the proximal target device 5 via the spacer 2.
Referring to FIG. 1, assuming that the center of rotation of the nail 6 is located essentially in the area of the Herzog curvature 7, then the nail 6 and the spacer 2 have centers of rotation situated in different planes, wherein the planes are at a distance from one another corresponding to the length of the spacer 1.
It can be seen from FIG. 2 that when the target device and nail are deflected by the angle .alpha., the center axes of the bores in the distal target device move away from the correspondingly aligned center axes of the channels in the medullary nail by a distance x2 lying further toward the end of the nail. As a result, the channels are no longer aligned with inserted locking screws when a nail is bent (pivoted).